The study will begin with a screening evaluation to ensure that each subject satisfies all eligibility criteria and to allow subjects to become familiar with our assessment procedures. Randomization of medications contained in look-alike capsules will occur at the baseline visit and is performed by a member of the Research Pharmacy staff. Patients will be evaluated at baseline, have direct formal scheduled evaluations at the end of treatment week's 2, 4, and 6 (+ 2 days), have telephone evaluations at the end of treatment week's 1, 3, and 5 (+ 2 days), and a final visit at 8 weeks. The primary outcome measure is effective tic suppression as determined by the difference in the Total Tic subscale (TTS) scores of the Yale Global Tic Severity Scale (YGTSS) at baseline and 6 weeks. Secondary outcome measures will include the change from baseline and 6 week scores for the YGTSS total score, the Clinical Global Impression -Improvement (CGI-I), and Patient Global Impression of Improvement (PGI-I). Secondary outcome for obsessive-compulsive behaviors will be measured by changes in the Children's Yale-Brown Obsessive-Compulsive Scale (CY-BOCS). Safety measures will include the use of an expanded Pittsburgh Side Effect Scale modified to include side effects of riluzole and D-serine, incidence of adverse events, measurement of vital signs (BP, pulse) and body weight, physical examination, laboratory studies (comprehensive metabolic panel (CMP), complete blood count (CBC), plasma amino acids, and urine analysis), the DuPaul ADHD Rating Scale, Child Depression Inventory-short version (CDI-S), and Multi-Dimensional Anxiety Scale for Children (MASC). A Drug Safety Monitoring Board containing three clinician-scientists with experience in pharmacological trials will meet quarterly to review this study.